Good but hamstrings seem to reoccur very very often once you get one.
Look at guys like Smalling, Fabregas had the issue for over a season at Arsenal before he left and got over it shortly after being at Barca. Messi did his sometime last Feb/march or so and it troubled him and effected his game right through to the world cup final. He finally at the weekend looked like the old Messi, not the goals but the movement throughout. He was involved over 90 minutes which he hasn't been since the injury, he was chasing players down and sprinting much much more frequently.
Ronaldo did his hamstring and like Messi it's hurting his overall performances and causing him to miss games aswell.
Costa did his hamstring in what April or so, they rushed him back and he broke down in the final games of the season, was no where near ready. Managed to play for Spain and last the games but looked no where near as mobile or energetic as usual, same as Messi/Ronaldo.
It seems to take anything from 6-18 months for a hamstring to get to the point that you aren't constantly reinjuring it and it's not effecting your game while you aren't playing.
Costa was a risk in that sense, he looked to have good movement in preseason/first two games, but they probably should have rested him more. I think it's where after it reoccurs 2-3 times you get the player starting to play more carefully which makes them play worse... then they get reinjured anyway.
In that sense Chelsea needed a quality back up, though I think Fabregas could be a great option there. Good goals/shots ratio for Barca,(excellent in two seasons, decent last season), the only thing stopping him scoring 20+ a year in each season was that he only took 45-55 shots, his job was still primarily to give the ball to others. He can do the same role though, bring in all of Chelsea's wide/central attacking players and set them up with chance after chance, or take more chances himself.